1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with an alignment layer including materials having a photosensitivity.
2. Description of the Related Art
It is generally known that a liquid crystal consists of anisotropic molecules. The average direction of the long axes of liquid crystal molecules is called the director of the liquid crystal. The director distribution in the liquid crystal is determined by the anchoring energy on a substrate, and is characterized by a director corresponding to a minimum of the surface energy of the liquid crystal and the anchoring energy. The director is rearranged by an electric field generated during operation of a liquid crystal display device (LCD). A LCD comprises two substrates opposed having liquid crystal therebetween.
In general, to obtain uniform brightness and a high contrast ratio, it is necessary to align the liquid crystal molecules uniformly in the liquid crystal cell. Several techniques have been proposed using polymers to obtain single or mono-domain homogeneous alignment of liquid crystal molecules. Particularly, it is known that polyimide or polysiloxane-based materials have high quality and good thermostability.
The most common technique employed as an alignment method to obtain a mono-domain liquid crystal cell involves forming microgrooves on the surface of the alignment polymer, which provides strong anchoring and stable alignment. In the above-mentioned technique, known as the rubbing method, a substrate coated with an alignment polymer is rubbed with a cloth. The rubbing method is a good method which can be applied to large scale LCDs, and thus is widely used in the industry.
The rubbing method, however, has several serious drawbacks. Because the shape of the microgrooves formed on the alignment layer depends on the rubbing cloth and rubbing intensity, the resulting alignment of the liquid crystal is often heterogeneous, causing phase distortion and light scattering. Further, an electrostatic discharge (ESD) generated by rubbing of the polymer surface further generates dust contamination in an active matrix LCD panel, decreasing production yield and damaging the substrate.
In order to solve these problems, a photo-alignment method has been proposed using a polarized ultraviolet light irradiated onto a photosensitive polymer to photo-polymerize the polymer (A. Dyadyusha, V. Kozenkov et al., Ukr. Fiz. Zhurn., 36 (1991) 1059; W. M. Gibbons et al., Nature, 351 (1991) 49; M. Schadt et al., Jpn. J. Appl. Phys., 31 (1992) 2155; T. Ya. Marusii & Yu. A. Reznikov, Mol. Mat., 3 (1993) 161; EP 0525478; and U.S. Pat. No. 5,538,823—a polyvinyl-fluoro cinnamate patent). The alignment capability of the photosensitive polymer is determined by the anisotropy of the photosensitive polymer, which is induced by ultraviolet light irradiation.
In the photo-alignment method, an alignment layer is given an alignment direction by irradiating a substrate coated with a photo-alignment material with a linearly polarized UV light. The photo-alignment layer comprises a polyvinyl cinnamate-based (PVCN) polymer, and as linearly polarized UV light is irradiated, the polymer photo-polymerizes through cross-linking. Cross-linking is generated among the polymers by the UV light energy.
In terms of the direction of the photo-polymers, the alignment direction of the photo-alignment layer has a specific direction in relation to the polymerization direction of the linearly polarized UV light. The alignment direction of the photo-alignment layer is determined by the direction of the photo-polymers. The pretilt angle of the photo-alignment layer is determined by the incident direction and the irradiating energy of the irradiated UV light. That is to say, the pretilt angle direction and the pretilt angle of the photo-alignment layer are determined by the polarized direction and the irradiating energy of the irradiated UV light.
With regard to photo-alignment, a polarizer is rotated in an arbitrary angle on each domain of the LCD. Then, in response to irradiating UV light, the polarization direction is changed, whereby a multi-domain LCD cell is achieved with multiple domains having different alignment directions in relation to each other.
The photo-alignment method, however, has several drawbacks. For example, it is impossible to apply on a wide scope. Most importantly, low photosensitivity of the photo-alignment material results in reduction of anisotropy and thermostability.
UV light irradiation takes a long time using conventional techniques, from approximately 5 to as long as 10 minutes. Low photosensitivity and small anisotropy make the anchoring energy of the final photo-alignment layer weak. Moreover, when the liquid crystal is injected into the liquid crystal panel, it is required that the injection be made at a high temperature. Low thermostability induces a flowing effect on the substrates, which can be observed as a ripple pattern in the liquid crystal upon injection between the substrates. Finally, disclination owing to the non-uniform alignment of liquid crystals remains as a problem to be solved.